Support per-slide centering

If config.center is false, this change centers slides with the
'center' class.
Added support for postMessage's into any iframes for content that needs to do something when a slide is displayed and hidden.
build latest js, format tweaks
apply @knownasilya's first slide detection fix to dev branch #671
contrib update
merge vertical centering for individual slides
Merge branch 'dev' of github.com:hakimel/reveal.js into dev
documentation for iframe post messages
kill the shadow on table headers